A fiber optic gyroscope (FOG) uses the interference of light to measure angular velocity. A FOG consists of a large coil of optical fiber. To measure rotation, two light beams are fired into the coil in opposite directions. If the sensor is undergoing a rotation then the beam traveling in the direction of rotation will experience a longer path to the other end of the fiber than the beam traveling against the rotation. This is known as the Sagnac effect. When the beams exit the fiber they are combined, and the phase shift introduced due to the Sagnac effect causes the beams to interfere, resulting in a combined beam whose intensity depends on the angular velocity.
Angle random walk (ARW) is one of the key performance parameters of a FOG. ARW is random measurement noise that ultimately limits the accuracy of the FOG and leads to navigation errors. A gyroscope's ARW can be specified by the manufacturer, but it can degrade over time due to faults, aging, or other causes.